Recovery of titanium metal



United tates Patent RECOVERY OF TITANIUM METAL Theodore D. McKinley, Avondale, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application April 10, 1952, Serial No. 281,653

2 Claims. (Cl. 75-121) This invention relates to the preparation of titanium metal and more particularly to novel methods for recovering titanium metal from a reaction product mass.

The reaction between titanium tetrachloride and magnesium at elevated temperatures and inert atmosphere yields titanium metal and magnesium chloride by-product salt. In practice, however, all reactants are not consumed and the resulting reaction product mass comprises titanium metal, usually in a spongy form, magnesium chloride, a considerable amount of unreacted magnesium metal, and some lower chlorides of titanium. Procedures used for recovering the titanium metal from the product mass have depended upon one of two properties of the materials to be removed, that is, volatility or solubility. Methods which utilize the volatile property of the byproduct and contaminating materials, that is, of magnesium chloride and magnesium, for separation purposes by distillation, require high temperature vacuum-type apparatus in which expensively constructed equipment, especially the vacuum pumping facilities, must be employed. Their operation also requires a considerable amount of operators attention and skill in assembling and operating the equipment. One major drawback in such processes is that any contamination of the reduction product mass with moisture, oxygen, or nitrogen, e. g., moist air, will result in undesired retention of these materials in the product metal mass. Since magnesium chloride is bygroscopic, there is a great tendency for this material to pick up moisture from air and hydrolyze to an oxychloride structure which induces high contamination of the titanium metal should air contact therewith be permitted. These oxide materials, because of their low volatility, are not removed during the distillation. Consequently, they are retained in and also react with the titanium metal at the distillation temperatures and disadvantage ously result in production of a hard brittle metal having little commercial utility.

Methods in which recourse to leaching of the reaction product mass is had to remove the magnesium chloride by-product salt, unreacted magnesium and contaminating lower chlorides of titanium require simpler equipment and do not have the drawback of having to limit the contact of the metal reaction product in the cold state with the atmosphere. However, in these procedures the metal product is objectionably and seriously attacked by the acidic solutions used in the leaching operation and a great loss of metal product results. This arises from the relatively high surface area of the titanium metal sponge. Also, the titanium metal product is degraded by the inclusion ,of hydrolyzed titanium and magnesium salts should dilute concentrations of acid be used in attempts at minimizing solution of the titanium metal.

It is among the objects of this invention to overcome the above and other disadvantages which have characterized prior methods for recovering titanium metal from its reaction product impurities and to provide novel and effective methods for attaining such objects. One principal object is to enable one to readily prepare pure titanium metal by recovery from a reaction product mass resulting from the reduction of titanium tetrachloride by magnesium metal. A specific object is to recover titanium metal from a reaction product mass comprising titanium metal, lower chlorides of titanium, unreacted magnesium, and magnesium chloride. Another object is to recover titanium metal from a reaction product mass by a process which combines the desirable features of low initial investment, low operating and maintenance cost and which yields consistently good product quality. Additional objects and advantages of the invention will be apparent from the ensuing description.

These and other objects are attainable by this invention which comprises subjecting an impure titanium metal reaction product to a leaching treatment with an aqueous sulfuric acid solution and in the presence of an inhibitor adapted to retard solution of the titanium metal, washing the leached product free of acid, and then drying and recovering the purified titanium metal.

In a more specific and preferred embodiment, the invention comprises freeing a titanium metal product from the reduction of titanium tetrachloride from magnesium chloride and magnesium impurities by subjecting the product mass in subdivided form to leaching with aqueous sulfuric acid solution containing an amino type inhibitor and an anti-foaming agent, continuing said leaching operation until the mass is rendered substantially free of magnesium and magnesium chloride impurities, separating the metallic mass from the treating solution, washing the separated mass with water until it is substantially free of acid, separating the acid-free metallic mass from the wash water, subjecting it to a further washing treatment with a water-miscible, volatile organic solvent until it becomes substantially free of water, removing the organic solvent from the water-freed mass and drying the resulting purified titanium metal.

In practically adapting the invention, any raw crude or partially purified titanium metal reaction product such as results from a conventional reduction process can be subjected to treatment. Thus, the crude metal product, together with residual magnesium chloride, unreacted magnesium, and any lower chlorides of titanium from the reduction of a titanium halide, such as titanium tetrachloride, with a reducing metal, such as magnesium, can be purified in accordance with the invention. Preferably, the crude comprises one from which the bulk of byproduct magnesium chloride salt has been removed by intermittent or continuous drainage during or subsequent to the reduction step and which in addition is in subdivided or particulate form from mechanical or other disintegration treatment, as by lathe turning, milling, or shearing, etc. Its size range is variable and suitable sizing can occur either in the reduction step or subsequently, as indicated, by subjecting large spongy masses to mechanical or other breaking up. Optimum particle size will depend upon and be governed by product porosity. Thus, in the case of very light porous spongy products, a 1" size will prove adequate for most leaching treatments, whereas with a relatively dense type of sponge more desirably a smaller size material comprising A" or smaller can be employed.

The finely divided reaction product together with its residual by-product, magnesium, lower chlorides and oxychlorides content is then charged into a suitable leaching vessel such as a corrosion-resistant tank or other equipment composed of metal, wood, or acid-proof brick lined, etc., containing preferably a dilute, say from 5- 25%, concentration of sulfuric acid to which has been added a relatively small amount, say from 0.05 to 1%, of an organic inhibitor which will protect the titanium metal from acid attack and allow the magnesium, magnesium chloride, lower chlorides, and oxychlorides to be preferentially dissolved. One particularly useful type of inhibitor comprises the reaction products of ethylene oxide with a primary rosin amine containing at least 10 carbon atoms for each nitrogen atom. Compounds of this type are more particularly described in U. S. Patents 2,510,063, 2,510,284, and 2,510,295. The molar ratio of ethylene oxide to rosin amine in these materials can be varied from 2-30. Since a larger amount of ethylene oxide has a solubilizing eifect, whereas borderline solubility is desirable for good inhibiting action, I find it advantageous in the case of the higher ratios to add from about 5-25 free rosin amine to the ethylene oxide-rosin amine reaction product. In mixing the inhibitor with the sulfuric acid, it will be found desirable to limit the rate of combination so that the heat of solution and reaction will not cause decomposition of the inhibitor or severe reaction with the strong acid present.

The surface activity of the type of inhibitors mentioned in conjunction with the evolution of hydrogen will cause excessive foaming of a very stable nature. To avoid this, while leaving sufficient unstable foam to prevent acid spray, I prefer to incorporate on the surface of the acid solution a suitable anti-foaming agent. One preferred form of foaming depressant for use in conjunction with sulfuric acid comprises a small amount of a mixed polymer of polyethylene and polypropylene glycols and in amounts ranging from, say, 0.005% to 0.1% on the basis of acid weight and depending on the rate of hydrogen evolution encountered and the surface area of the acid being employed.

Some movement of the solvent acid solution relative to the metal product will prove desirable in order to promote acid action by either scrubbing ofi adherent hydrogen bubbles which tend to blanket the material under treatment and/or increasing the diffusion of fresh acid into small crevices and pores. This may be accomplished by suitable mechanical stirring or tumbling of the product during the leaching operation and by means of stirrers or paddle means suitably associated with the leaching tank and designed to promote circulation and mixture of the product undergoing treatment with the leaching solution. Alternatively, and particularly in instances where a soft spongy product metal is undergoing treatment, I prefer to circulate the solvent acid, thus avoiding the mechanical closure of pores which might lead to incomplete extraction. As a safety precaution, a hydrogen venting and disposal system can also be associated with the leaching equipment.

The time consumed or required for effecting complete extraction is subject to a great number of variables and will depend upon the porosity and state of subdivision of the material undergoing treatment. When extraction is complete, the metal product is removed from the acid and is washed with water, acidified (to prevent hydrolysis of salts on dilution) to a pH of 1-2 in order to eifect removal of the occluded salts. The product mass is then washed with fresh water to remove all traces of acid. Drying of the metal product after removal from the wash water is then undertaken which may be done in the presence of air, but if the product is a sponge of large surface area, a temperature of 60 C. should not be exceeded in the drying operation so that surface oxidation of the metal will not take place. Alternatively and preferably, such drying procedure is effected by washing the purified sponge metal with an organic solvent such as methanol or other suitable water-miscible, volatile solvent until its water content is removed. Thereafter, the excess methanol is drained from the product and the latter finally dried at a temperature below 60 C. and within, preferably, a temperature range of from 4050 C. Resort to solvent drying is especially desirable and effective where removal of all traces of inhibitor from the final product is to be effected. The acid extract from the leaching step can be suitably treated to remove dissolved product salts or concentrated to yield hydrated salts of magnesium or can be subjected to suitable recovery treatments as desired in accordance with well-known commercial techniques.

The titanium metal product recovered will be found to be in relatively pure condition and substantially free from objectionable impurities. If desired, it can be employed directly in various commercial applications or can be subjected to a vacuum distillation treatment to further reduce or eliminate traces of impurity present which might be undesirable for particular applications of the metal.

To a more complete understanding of the invention, the following specific examples are given. These are illustrative only and are not to be considered as in limitation of my invention:

Example I A crude titanium metal product, weighing 145 lbs. and containing 12.4% unreacted magnesium and 19.9% magnesium chloride residue, recovered from the reduction of TiCl-t with Mg, was placed in a milling machine from which chips, about 4-10 mesh in size, removed from the mass, were recovered for treatment. These chips (which had been exposed to air for some time and had a water content of about 1.5%) were added to a tank containing 70 gallons of 10% sulfuric acid and 1.5 lbs (about .25%) of an inhibitor composed of 85 parts of rosin amineethylene oxide reaction product having a ratio of 5 mols of ethylene oxide per mol of rosin amine with a 15% content of free rosin amine. In addition, a .05% content or 0.3 lb. of an anti-foaming agent consisting of copolymerized polyethylene and polypropylene glycols was utilized on the surface of the sulfuric acid. The chips were added to the pool of sulfuric acid at a rate to insure that the temperature of the acid bath and contents did not go above 40" C. and for the greater part of the time remained within the range 2530 C. The metal product was allowed to remain in the acid bath for a 24-hour period. At the end of this period, the acid was removed from the metal by draining and through opening of a valve in the bottom of the tank. The treated metal was then subjected to a rinsing treatment by filling the tank up several times with a very dilute solution of sulfuric acid of pH 1-2 and draining. Then the acid so lution was replaced with water and the tank drained, this latter operation being repeated until the wash water drained from the leaching tank was substantially free of acid. The metallic mass was then further washed with methanol until substantially water-free. After being freed from the excess methanol, the titanium metal was dried in trays in an oven at a temperature not in excess of 60 C.

The titanium metal product from this operation upon analysis was found to contain about .44% residual magnesium and .14% chlorine, the amount of titanium lost during the leaching treatment being about 0.35%, determined by analyzing the amount of titanium in the acid solutions. The Vickers Hardness of the product at this stage on a button melted in an arc furnace was 170. In a comparable operation in which the above process was duplicated except that no inhibitor or anti-foaming agents were used in the leaching operation, there resulted a 20% loss of titanium metal by solution into the acid leaching reagent.

Upon subjecting the product from my improved method above to a vacuum distillation treatment, a further improvement in the quality of the titanium metal produced can be realized. When such product was heated to a temperature of from 875-925 C. for a period of 8 hours in a vacuum distillation retort in which the ultimate vacuum utilized was 40 microns, the product recovered, on analysis, contained .03% magnesium and .13% chlorine. An arc-melted test button of this titanium product exhibited a Vickers hardness number of 168.

Example II A titanium metal sponge reaction product from the reduction of titanium tetrachloride with magnesium metal in accordance with the disclosure of U. S. Patent 2,205,854 was first heated in a separation retort to drain off and remove the major portion of its magnesium chloride by-product salt impurity. The pretreated product analyzed 71.96% titanium, 11.8% magnesium metal, and 14.4% magnesium chloride. This material was then reduced to lumps of less than 1 inch in size by chucking it in a lathe and breaking it up with a rough tooling operation. parts by weight of 10% sulfuric acid containing .25% by weight of solution of an inhibitor and .05% by weight of an anti-foaming agent of the same types used in Example I, were placed in a corrosionresistant container. To this inhibited acid solution were added 10 parts by weight of said titanium metal lumps, the rate of addition being regulated so that the temperature of the acid solution did not exceed 50 C. The material was allowed to stand in said acid solution for a total of 24 hours. Thereupon, the leached metallic mass was separated from the acid solution and washed with water and methanol as in Example I. It was then dried in an oven utilizing a current of air and a temperature not in excess of 60 C. The purified titanium metal product recovered was found to be substantially free from magnesium and magnesium chloride impurities and was of satisfactory quality.

My leaching process is unique in that use of the inhibitors mentioned renders it possible to protect one acidsoluble metal (titanium) while dissolving another (magnesium) even though the two metals are in finely divided form and in intimate contact. Thus, advantageously, magnesium removal can be effected to approximately its limit of solubility in titanium, that is, about 0.4% magnesium in the metallic titanium.

One of the novel features of my invention is that during this operation of subdividing the product mass it is totally unnecessary to exclude air or moisture from the materials. This is in contrast to the other processes using heat and vacuum or heat alone for removing residual magnesium chloride and magnesium in that in these processes the introduction of moisture before and during removal of magnesium chloride and lower chlorides must be diligently guarded against since moisture pick-up would invariably lead to oxygen contamination of the finished product. The partial or total exclusion of moisture may be practiced at this step in my invention, in the interest of preserving machinery, but whether or not this is done will be of insignificant import to the final quality of the product.

Although the invention has been'described as applied to certain specific embodiments, it obviously is not restricted thereto. The inhibitors mentioned are preferred for use because they prevent titanium attack even for quite lengthy periods, as shown by the following test: In a static leaching test after 8 hours 0.2% titanium was dissolved, after a 24-hour period 0.35% titanium had been dissolved; and after 143 hours 0.96% titanium had been dissolved. In general, with the preferred operating techniques in the leaching step the l43-hour test would be much too lengthy for a period of contact between the acid and the metal and the preferred range would be more of the 24-hour-or-less contact period. Other inhibiting agents, especially long-chain complex amine compounds such as alkyl or alkaryl amines with a chain length of or more carbon atoms, are also contemplated as utilizable. However, these do not aflord optimum results, being less effective, and hence are not preferred for use in my invention. Examples of such additionally useful inhibitors include H(C2H4NH)14NH2,

and tridecylbenzyl hydroxyethyl imidazolinium chloride, etc. The titanium metal protection in such instances is only partial in nature and more of it is lost by solution while the magnesium, magnesium chloride, oxychloride, and lower chlorides of titanium are dissolved. Many of these inhibitors comprise commercial products of mixed composition, sold only under trade names. In general, it may be considered that an inhibitor is beneficially useful and effective herein if the amount of titanium metal dissolved during the leaching operation is less than about 5% while allowing the magnesium, magnesium chloride, and other leachable material contents to be substantially removed.

The amount of inhibitor to be used will vary with its composition, the nature and strength of the acid solution, the surface area of the metal product, and ratio of acid to metal, etc. However, the optimum level does not exceed 3 of the acid solution by weight. Once there is suflicient inhibitor present to inactivate the active centers of the product, no further benefit is derived from the use of a greater quantity. Thus, when using 10% sulfuric acid to extract magnesium, by-product magnesium chloride, lower chlorides and oxychlorides from a sponge product mass from 0.1 to 0.5%, based on the weight of acid solution utilized, of the preferred form of inhibitor will prove sufficient to allow essentially complete removal of the impurities mentioned, while dissolving less than 0.5% of the titanium metal. As indicated, a preferred inhibitor comprises a mixture of 85 parts of ethylene oxide-rosin amine in which the molar ratio is 5, and parts of free rosin amine. An alternate inhibitor com prises a mixture of 9 parts ethylene oxide-rosin amine reaction product, in which the ethylene oxide to rosin amine molar ratio is 11, and one part free rosin amine.

A certain number of factors is to be considered in the leaching step, the following general items being useful as guides in obtaining optimum results. Thus, while sulfuric acid in dilute, non oxidizing concentrations comprises a preferred acid leaching agent because of economic reasons, other acids including acetic acid are also operatively useful. The amount of acid to be used will depend upon the size of the batch under treatment and also upon the magnesium metal content of the crude product being treated. The preferred acid strength lies in the range of between about 2-15% sulfuric acid by weight of the solution. Below about 2% sulfuric acid hydrolysis of magnesium and titanium salts may occur and also the solution rate of magnesium in this weak acid becomes quite slow. Concentrations of acid greater than 15% by weight allow a slightly more rapid rate of titamum metal corrosion and therefore would lead to slightly higher losses of product metal into the leaching solution. During the leaching operation it is preferred to keep the temperature of the batch and solution less than 50 C. and to within a range of from 35-45 C. to moderate the solution rate of the titanium metal and prevent inhibitor decomposition. This temperature can be regulated by controlling the rate of introduction of the crude metal into the acid leaching bath, or by cooling the leaching apparatus by well-known means. Circulation of the acid leaching solution or stirring of the mass are preferred as aids in keeping the temperature of the center of the mass down, for securing uniform acid concentration, and also may aid somewhat in the diffusion of the reacting materials.

The various washing operations following complete extraction of impurities can be effected in well-known means, such as washing centrifuges, basket container for charge, open type of draining filters, etc., and with water acidified to a 14 pH to remove occluded salts, followed by water washing of the product to remove all acid. The final methyl alcohol or other volatile organic solvent rinse is useful in removing the traces of inhibitor from the titanium metal. It is preferred to do the drying operations on the titanium metal product at a temperature below 60 C. and within a range of from 3550 C. to minimize the amount of surface oxidation of the purified titanium metal. Recourse to the vacuum distillation following leaching will also help to remove some dissolved magnesium and drops the magnesium content of the purified metal to about 0.02% magnesium from the range of about 0.40% magnesium obtained in the leached product as well as to aid in removing occluded hydrogen.

This additional step provides a titanium metal which is less gassy during a melting operation and results in ingot castings having fewer gas pockets. This will be found to improve the mechanical properties of the metal since it is considered that occluded hydrogen presence is detrimental.

It is obvious that any reduction operation of titanium tetrachloride with magnesium may be utilized to prepare a crude product for the leaching steps. As noted, it is desirable to remove a major portion of the magnesium chloride by-product during or subsequent to the reduction step. Many means for effecting this preliminary purification operation such as draining during or subsequent to the reaction, leaching of the magnesium chloride from the product mass, using a non-aqueous solvent such as propanol, mechanical removal of a large amount of magnesium chloride, etc., can be resorted to. The next consideration is to determine that the crude metal reduction product is in the size range required for adequate leaching. The sizing of product can be effected in the reduction step and the desired particulate product obtained therein, or if produced in large spongy masses it can be broken up by mechanical or other disintegration treatment.

From the above, it will be apparent that my novel process combines the desirable features of low initial investment, low operating and maintenance cost, and insures production of a product of consistently good quality; also, that the process is generally adapted to the treatment of metal reaction products from many metal reduction processes, especially those in which a chloride or other operable halide (bromide, iodide) of the metal under production is reduced at elevated temperatures with magnesium or other suitable type active reducing metal. Thus, in addition to Ti, purification is contemplated of impure or by-product salt-contaminated refractory metal reaction products of Zr, Be, Hf, Cr, V, U, Th, Ta, W, Mo, etc., and especially those from the reduction procedures of U. S. Patents 2,205,854, 2,171,439, 2,214,211, 2,148,345, 2,556,763, 2,567,838, or 2,564,337.

I claim as my invention:

1. A process for purifying a sponge titanium metal product containing magnesium as an impurity, and resulting from the reduction of titanium tetrachloride with magnesium, comprising selectively removing said magnesium impurity by leaching said product while in subdivided state with an aqueous sulfuric acid solution containing between 2l5% by weight of H2804 and from .05l%, based on said acid solution Weight, of an organo-amine inhibitor consisting of a rosin amine-ethylene oxide reaction product having 2-15 moles ethylene oxide per mole of primary rosin amine and 5-15 mole per cent of free primary rosin amine, eflecting said leaching at a temperature not in excess of about 50 C. and until said titanium metal product is rendered substantially free of impurities, washing the leached metal product free of acid, and lthen drying and recovering the purified titanium meta 2. A process for purifying a sponge titanium metal product containing magnesium as an impurity and obtained from the reduction of TiCLi with magnesium, comprising selectively removing said magnesium impurity by 10 ties, washing the leached metal product free of acid, drying the purified product by washing with methanol, and finagly drying said product at a temperature of from 40- 0 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,499,283 Robinson Feb. 28, 1950 2,510,063 Bried June 6, 1950 2,510,284 Haggard June 6, 1950 2,564,753 Cox Aug. 21, 1951 OTHER REFERENCES Mann, Lauer & Hultin, Organic Inhibitors of Corrosion, Ind. and Eng. Chem. 28, 159-163 (1936).

U. S. Air Force Project Rand, Titanium and Titaniumggise Alloys, published by The Rand Corp. (1939), page 

1. A PROCESS FOR PURIFYING A SPONGE TITANIUM METAL PRODUCT CONTAINING MAGNESIUM AS AN IMPURITY, AND RESULTING FROM THE REDUCTION OF TITANIUM TETRACHLORIDE WITH MAGNESIUM, COMPRISING SELECTIVELY REMOVING SAID MAGNESIUM IMPURITY BY LEACHING SAID PRODUCT WHILE IN SUBDIVIDED STATE WITH AN AQUEOUS SULFURIC ACID SOLUTION CONTAINING BETWEEN 2-15% BY WEIGHT OF H2S04 AND FROM 905-1%, BASED ON SAID ACID SOLUTION WEIGHT, OF AN ORGANO-AMINE INHIBITOR CONSISTING OF A ROSIN AMINE-ETHYLENE OXIDE REACTION PRODUCT HAVING 2-15 MOLES ETHYLENE OXIDE PER MOLE OF PRIMARY ROSIN AMINE AND 5-15 MOLE PER MOLE OF PRIMARY ROSIN AMINE, EFFECTING SAID LEACHING AT A TEMPERATURE NOT IN EXCESS OF ABOUT 50* C. AND UNTIL SAID TITANIUM METAL PRODUCT IS RENDERED SUBSTANTIALLY FREE OF IMPURITIES, WASHING THE LEACHED METAL PRODUCT FREE OF ACID, AND THEN DRYING AND RECOVERING THE PURIFIED TITANIUM METAL. 